sageleaf and gumix! I have something for you...
Coming to the next release of REXPaint is... you guessed it, customizable glyph mirroring!
It's already available now as a patch if you'd like access to it (or give any feedback) before the official release. You can download the
preview patch here(edit: patch removed since this is now included in 1.60), which includes only files that are new/modified from v1.50.
With this version you can assign glyph pairs on a per-font basis by specifying a file where those values can be found. Notice that the font _config.xt also includes two new columns to make new features possible. (This same patch also includes support for customizing Unicode code points, as well as a few other QoL features.)
The updated manual should have the necessary details:
...
Notice in _config.xt that each art font also includes two additional parameters: Unicode and Mirror. These specify two text files, also expected to be found in the "data/fonts/" directory, which define code points for UTF8 text exporting and glyph mirroring behavior for the paste tool, respectively. The default files provided are compatible with a standard CP437 layout. If neither of those features is required for a given font, using the default files is fine, or even just insert a single hypen ('-') for a filename there and it will be ignored. See the sections below for more information on customizing these features.
...
Custom Glyph Mirroring
------------------------
While the Paste tool's alternate mode can mirror CP437 glyphs, with custom fonts using some other layout it is often desirable to specify different sets of glyph pairs for this purpose. You can do this by creating a new text file in the "data/fonts/" directory that defines pairs and assign it to its respective font in _confit.xt via the Mirror column. See the default CP437 _mirror.txt file for an example of the required layout. Each line should specify a single pair, and indicate whether the mirroring direction is horizontal (h) or vertical (v). Indices start from 0 and proceed in row-major order, as displayed in the font bitmap. Add optional comments at the end of a line by preceding them with two slashes.
Here's a demo showing some custom mirroring with an extended tileset:
Let me know how it works out.